Motive or traction batteries, such as those designed for fork lifts or golf carts must be capable of providing steady power over an extended Period of time (such as 8 or more hours before recharging). They must also be able to be recharged without careful or frequent supervision.
These batteries are of the lead acid type having one or more positive plates and negative plates with a battery separator there between and an acid electrolyte, generally based upon sulphuric acid. Such batteries have traditionally used a Porous hard rubber (sulfur cross-linked rubber) separator between the lead plates. Such separators are particularly favored for traction batteries as they provide adequate strength and ionic flow with little dendritic growth over a long life span (up to 20 years in some instances). However, such separators have some drawbacks. Most importantly, such separators are difficult and costly to produce and their availability is somewhat limited due to few available suppliers. Moreover, the separators are heavy, brittle and relatively thick which reduce electrolyte storage capacity and increase battery weight.
Alternatives have been tried, but most have failed to gain widespread acceptance by the battery industry. One such alternative has been to use a microporous, phenolaldehyde resin impregnated paper sheet separator to one side of which a glass mat layer has been bonded. It however, has not been able to provide performance comparable to that of the hard rubber separators.
All separators used in these traction batteries suffer from several Problems during use, all of which tend to shorten the life and power of the battery.
The first Problem is known as "the top of charge" or "top of voltage" phenomenon. The typical battery charger used for traction batteries has no battery temperature or current sensor for determining when the battery is completely recharged. Rather, it is a simple device which supplies a high voltage to a battery until the operator or a timer causes the device to stop supplying the voltage. In some instances, the charger does not stop supplying the voltage before the battery is completely charged, thus causing the battery to become overcharged. This tends to occur more frequently as the age of the battery becomes greater. During an overcharge situation, the components of the battery, in particular the plates and the separators, become subjected to high temperatures which increase their suseptibiltiy to oxidation or "burning" and thereby shorten the battery's life.
Second, in those instances where the lead used in the battery plates contains antimony, the batteries are suseptible to antimony poisoning. Antimony is often a desirable component of the lead alloy used in forming the plates of the battery in that it forms harder, more acid resistant plates. Antimony, however, tends to dissolve from the positive plate, due to oxidation and other forces within the battery especially during overcharged conditions and travel through the separator to the negative plate where it plates out on the surface of the plate. The plated out antimony reduces the active surface area of the negative plate and hereby reduces the capacity and voltage behavior of the battery.
Lastly, dendritic growth occurs through the separator causing the adjoining plates to short.
The present invention provides an alternative to the known separators, which invention overcomes the problem of top of charge, antimony Poisoning and reduces the extent of dendritic growth.